Fossil fuels such as but not limiting to natural gas are formed deep inside the earth when layers of plants, animal matter and gases are exposed to intense heat and pressure for thousands of years. During transportation of these gases (along with formation water) from extraction wells to receiving facility such as onshore terminals, gas hydrates or clathrate hydrates are formed which tend to clog the flow lines such as pipes. The gas hydrates physically resemble ice and are formed due to entrainment of non-polar molecules (mostly gaseous molecules) in cages of hydrogen bonded water molecules to form crystalline water based solids, which cause blockages in flow lines. For many years, several hydrate inhibitors such as but not limiting to glycols and methanol have been used as primary chemical compounds to prevent blockages in pipe lines due to gas hydrate formation, and are usually injected into production fluids present in extraction wells.
Generally, onshore and offshore applications require persistent inhibition of hydrates, and hence, the cost of replacing hydrate inhibitor that is lost to the gas and liquid hydrocarbon product streams is a determining factor in selecting the inhibitor. Methanol solubility in gas and liquid hydrocarbon product streams may be two or more orders of magnitude higher than glycol solubility. This creates a strong economic motivation to use glycols such as but not limiting to Mono Ethylene Glycol (MEG), despite the greater quantity of MEG needed per degree of hydrate temperature suppression. However, the adoption of MEG over methanol has taken some time to occur, due in part to familiarity with methanol and owing to operating difficulties in recovering and recycling MEG.
Conventionally, MEG is used in hydrocarbon gas and/or condensate pipelines to absorb moisture and prevent gas hydrate formation in the pipeline, which otherwise can lead to blockage and corrosion. Typically, MEG (or other inhibitor) is injected into well fluids at a loading facility, and is separated from the well fluids at a receiving facility. The separated MEG (known as rich MEG), which carries absorbed water (containing formation water and condensed water) is regenerated by a water and salt removal process to produce “lean MEG” for re-use. The separated MEG also tends to become polluted by other components in the pipeline, such as pipeline corrosion products, dissolved gases, hydrocarbon condensates and salts. The salts are mainly present in formation water, and may separate by forming precipitates during the MEG regeneration process. Removal of these particles from MEG improves the performance of the MEG regeneration process, because the particles tend to accumulate in the regeneration process, and clogs process equipment. One known solution to this problem is to separate the particles by introducing a solid separation unit including but not limiting to a centrifuge in the MEG reclamation and regeneration plant.
Further, formation water present in rich MEG contains salts which include low solubility salts and high solubility salts. The low solubility salt precipitates include ingredients such as but not limiting to Calcium carbonate, Magnesium Hydroxide, Iron carbonate and so on, and are mostly divalent salts. Generally, some divalent cations like Ca2+, Fe2+, Mg2+, etc undergo ionic bonding with divalent anions to form insoluble salts whose solubility decrease with increase in temperature. Hence, it is necessary that these divalent salts are removed from rich MEG so that they do not proceed further to reclamation unit where they are rendered insoluble, under certain process conditions, such as rise in temperature. The high solubility salts, on the other hand, include but not limiting to Sodium chloride, Potassium chloride etc, which are mostly monovalent salts. As the name implies, these salts have high degree of solubility and may be regarded as dissolved impurities present in rich MEG solution. Now, MEG has to be separated from water and salts (monovalent and divalent) for re-injecting back to wells through MEG Pre-treatment and Reclamation plant. Conventional MEG reclamation and regeneration plants comprise of a separation unit which receives mixture of MEG and fuel in liquid and/or gaseous state (called well fluids) from extraction wells, through one or more flow lines. The separation unit separates MEG solution from the gas, and gas is sent to dehydration unit for removal of moisture and thereafter for further processing. The separated MEG i.e. rich MEG is fed into the regeneration unit, and a small part of rich MEG containing supersaturated precipitates of both the salts (monovalent and divalent) are sent to a solid removal unit such as but not limiting to a centrifuge including but not limiting to a decanter type centrifuge. The solid separation unit separates the supersaturated salt precipitates from incoming streams, and thereby allows clear MEG to flow into the regeneration unit for further processing. In the regeneration unit, moisture content, i.e. water, is removed from the MEG to obtain MEG in its lean form which can be re-injected into extraction wells to continue the cycle.
The presence of salts, and most importantly the divalent salts in rich MEG solution poses problems such as formation of scales (scaling) in flow lines and tend to choke few units of the reclamation section, and mainly the chamber containing centrifuge. This creates a need for manual intervention to periodically clean the reclamation unit as well as the centrifuge. In addition, precipitates of divalent salts tend to corrode pipelines and cause suspension of corroded particles, thereby increasing the concentration of suspended impurities in the MEG solution. This increases load on the centrifuge and hampers its performance, which in turn hampers the performance of MEG Reclamation Plant. In addition, high vibration and torque are observed during centrifuge operation due to difference in feed water concentration and increase in temperature. Hence, the system shut down time is more, resulting in inefficient utilization of resources, which are undesirable. Another problem encountered during the process is producing high quality lean MEG solution, without compromising with quantity of water removed from the rich MEG solution. A MEG solution which has salinity less than 500 ppm is considered to be of high purity. However, with the increase in water handling capacity, finer salt particles would be carried with the MEG solution into the reclamation column, which again results in one or more limitations as explained above.
In addition to the above, the conventional MEG regeneration systems can handle water only up to 300 m3/day. This results in reduction in efficiency of the system in terms of quality and quantity of regenerated MEG, which would inherently increase the process time and power consumption. However, if an attempt is made to increase the water handling capacity above 300 m3/day in the conventional MEG regeneration systems, the salinity and conductivity of the regenerated MEG will be increased. This makes the MEG impure, and poses problems as stated above, making it unsuitable for re-injection into extraction wells.
In light of foregoing discussion, there is a need to develop an improved system and method for regenerating mono ethylene glycol to overcome the limitations stated above.